1. Field of the Invention
The present invention relates to a laser shock processing operation, and, more particularly, to a method and apparatus for accurately and automatically tracking the position of a workpiece, such as an integrally bladed rotor, by dynamically adjusting the position of the workpiece in order to compensate for the presence of distortion or manufacturing variations.
2. Description of the Related Art
Laser shock processing, or laser shock peening, or laser peening, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser pulses directed onto the surface area of a workpiece. Laser shock processing is an effective method of increasing fatigue resistance in metals by treating fatigue critical regions. For a more thorough background in the prior history of laser shock processing, a reference can be made to U.S. Pat. Nos. 5,131,957 and 5,741,559, such patents are explicitly hereby incorporated by reference.
Laser shock processing, as understood in the art and used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a workpiece by producing an explosive force by instantaneous ablation or vaporization of a painted, coated, or un-coated surface. Laser peening has been utilized to create a compressively stressed layer in the subsurface of a workpiece, thereby considerably increasing the resistance of the workpiece to fatigue failure. Laser shock processing typically utilizes two overlays: a transparent overlay (usually water) and an opaque overlay, typically an oil-based, acrylic-based, or water-based paint or tape. Laser shock processing can also utilize only a transparent overlay or bare surface. During processing, a laser beam is directed to pass through the transparent overlay and is absorbed by the opaque overlay or bare surface, causing vaporization of a portion of the opaque overlay or bare surface, which results in rapid plasma formation and the generation of a high amplitude shock wave. The shock wave cold works the surface of the workpiece and creates compressive residual stresses, which provide an increase in fatigue properties of the part. A workpiece may be processed by producing a matrix of overlapping spots that cover the fatigue-critical zone of the part.
Laser shock processing is being used for many applications within gas turbine engines, such as leading and trailing edges of fan and compressor airfoils. These laser peening applications, as well as others, are in need of improved positioning techniques to reduce setup time and improve the quality and consistency of the processed part. The quality of laser peening depends, in part, upon the accurate and repeatable positioning of the laser beam on the part.
Current laser-beam-positioning methods for laser peening parts are accomplished by moving the workpiece to a definite, hard-coded position in space and then firing the laser. The index of this point in space for purposes of identifying the target area can be any convenient feature of the workpiece or part manipulator (reference point), e.g. a corner of the platform of an airfoil, not the coordinates of the point where the laser hits the part, i.e., target area. Consequently, as the reference point of the part (or subsequent similar parts) is moved to the same location, small deformations, distortion, and variations within dimensional tolerances in each individual part (specifically, at or near the target area) will change the exact point where the laser hits the part.
When laser peening thin sections, such gas turbine engine blades, it is usually desirable to use two-sided processing methods and maintain the symmetry of the shockwaves in order to most efficiently and effectively laser peen the part. Typically, matching the shockwaves generated on opposite sides of a thin section is accomplished by maintaining a substantially identical laser spot size and shape on opposite sides of the part for each laser pulse within the spot pattern being processed. If the angles at which the laser beams are presented to the part are maintained as congruent, and the reference point for the part (which can be many inches away from the target area where processing is to occur) is held as constant, small deformations due to previous processing or dissimilarities between parts can cause the laser to hit the part in an asymmetric manner causing improper processing.
Because of the compressive residual stresses imparted by laser peening, small distortions in the part can occur, especially in thin airfoils. Under circumstances where a first series of laser-peened spots causes a slight deformation in the part, the application of a second series of spots over substantially the same target area may result in the beams no longer being disposed substantially opposite one another, depending upon the amount of distortion and the angle on incidence of the laser beams. If the part-positioning program is hard-coded, the operator may never be aware that subsequent series of laser-peened spots were misaligned. These types of in-process misalignment problems can lead to significant variation in the quality and performance of the laser-peened parts, without the operator even realizing the source of the variability.
According to the present invention there is provided a method for processing a workpiece whereby the laser peening system (or associated hardware or software) automatically detects, and then automatically compensates for, deviations from the ideal positioning of a part. If all parts were identical, the part manipulator could be preprogrammed for the ideal part and each part would then be processed identically, using the same program. However, deviations from the ideal part occur.
By way of background, such deviations or departures from an ideal construction may stem from normal manufacturing tolerances, tolerance or repeatability problems associated with the part fixture that holds the part in the manipulator, distortions caused by earlier laser peening steps on the part, or any other effect that would cause the part to not be positioned in an ideal or pre-determined location relative the laser beam.
Dissimilarities can exist between the workpiece being processed and the template or test workpiece that was used to derive the part processing program. Due to a lack of exact reproducibility in the manufacturing process, the same manufacturing operation can produce a series of workpieces of the same type that have dimensional variations relative to the ideal workpiece, yet still be within manufacturing tolerances. The problem that arises relates to the fact that the part program for controlling the positional movements of the workpieces during laser shock processing is based upon the ideal part construction or a test part that was used during programming; accordingly, any dimensional or structural deviations can result in the laser beam impinging upon the production workpiece in a manner or place different from that contemplated in regard to the ideal workpiece.
Another issue that adversely affects the reliability and repeatability of laser peening arises from the fixturing of the part within the part manipulator. Slight misalignments that may occur during mounting of the part, which may be caused by normal tolerance problems or human errors, can lead to significant misalignment of the laser beam on the part.
Still another issue that adversely affects the reliability and repeatability of laser peening relates to distortion effects that can arise during laser peening because of the compressive residual stresses imparted by the process. Consequently, it is possible that the alignment for subsequent laser peening sequences on a part may be adversely affected by preceding laser-peening sequences. Even if the distortion of the part remains within manufacturing tolerances, the subsequent laser-peening sequences may be ineffectual or even deleterious, if processing continues after a misalignment has occurred.
In view of the foregoing, there is proposed herein a method and apparatus that overcome the disadvantages found in conventional laser shock processing operations.
The processing method of the present invention involves providing a conventional part program that defines a sequence of fixed processing positions, i.e. a preprogrammed spot pattern. At the outset of the processing operation, the workpiece is moved to an initial or first processing position in the part program sequence. Measurements are taken to collect and otherwise acquire workpiece data that defines a spatial parameter characterizing a current target area of the workpiece.
In one form, the spatial parameter represents the distance between the current target area and a reference point such as the base which supports the workpiece. Alternatively, the spatial parameter can define a target area profile which provides a geometric representation of the current workpiece target area. In all cases, the spatial parameter facilitates a comparison between the actual position of the current target area and the ideal position of the same target area as determined in connection with the ideal workpiece construction upon which the part program is based.
The collected workpiece data is processed and otherwise analyzed to evaluate the spatial parameter in relation to predetermined criteria. For example, when the spatial parameter represents a distance measurement, the spatial parameter is compared to a reference distance value that was obtained in connection with the ideal workpiece. Based upon this comparison, a difference value can be obtained which represents the variation of the production workpiece measurement from the ideal workpiece measurement. The position of the workpiece is then adjusted in accordance with the evaluation results to enable reliable, repeatable, and reproducible laser shock processing of the workpiece at the current target area.
After the laser shock processing operation is performed on the workpiece at the current target area, the workpiece is moved to the next sequential target area of the processing positions and the aforecited processing method is repeated until the part program is completed.
The workpiece is typically laser peened by processing a matrix of overlapping or non overlapping laser beam spots that cover a critical zone of interest. Additionally, the same or adjacent areas may be repeatedly processed by cyclically directing the laser pulse to the desired target area. Various parameters may be controlled by the production manager to tailor the laser shock processing operation. For example, among the operational parameters that the designer can select and adjust include (but are not limited to) the location of the incident beam spot, the number of spots at each location, spacing between spots, distance of spots from or to certain workpiece features (e.g., leading and trailing edge of integrally bladed rotor), angle of incidence of the laser beam, and the laser beam metrics (energy, pulse risetime, pulse width, spot shape, etc.). Additional descriptions may be found in U.S. Pat. Nos. 5,741,559 and 5,911,890, both assigned to the same assignee as the present application and incorporated herein by reference thereto.
One significant advantage of laser shock processing is its ability to increase the fatigue properties of the part by selectively imparting compressive residual stresses within certain critical areas where incipient weaknesses or cracks typically appear. The technique has been applied with favorable success to the processing of the pressure and suction sides of leading and trailing edges of fan and compressor airfoils and blades in gas turbine engines.
As used herein, a workpiece refers to any solid body or other suitable material composition that is capable of being treated by laser shock processing. The workpiece may represent a constituent piece forming part of an in-production assembly, a final production article, or any other desired part. Accordingly, the laser shock processing treatment may be applied at any stage of production, i.e., pre- or post-manufacturing or any intervening time. Preferably, in certain industrial applications, the present invention finds significant use in processing the airfoils of an integrally bladed rotor, most notably in the region proximate the leading and trailing edges where flaws and cyclical fatigue failures pose serious problems affecting the performance and durability of the engine.
As used herein, a part program conventionally refers to the sequence of positions where the workpiece is located during each interval or stage of laser shock processing. Typically, the workpiece (or its assembly) is loaded into a part manipulator or other such machine of conventional construction having a control apparatus implemented by a microprocessor. This computing device is preprogrammed with the part program, which contains a predetermined set of instructions representing the various locations where the workpiece is to be positioned and the timing and sequence in which such movements are to take place. The movement of the workpiece is coordinated and otherwise synchronized with the operation of the laser apparatus using a suitable timing and control apparatus or other suitable system management facility.
The part program is typically accompanied by or includes a laser operation program that serves to link or otherwise associate the various workpiece processing positions with corresponding laser shock peening activity characterized by parameters including, but not limited to, pulse number and intensity, angle of incidence, laser-beam spot size, laser-beam spot shape, pulse duration, pulse reforming or reshaping, and pulse modulation.
As used herein, optimal processing refers generally to any form of laser shock processing that produces a desired outcome or result. This result, for example, may be measured or determined by whether the processed article and/or the spot pattern exhibits, meets, or otherwise satisfies predetermined criteria formulated by the designer.
A general aim of such optimization involves the development of shock-induced compressive residual stresses without introducing any distortion into the workpiece. Alternately, this optimization may be considered to involve the elimination or substantial reduction in the possibility of non-uniformly working the material stemming from a non-uniform application of energy to the workpiece. This non-uniformity may be characterized in a number of ways, for example, asymmetrical shock-induced stress regions, mismatched or unbalanced shock wave activity, misalignment of laser beam spots impacting opposing sides of a workpiece, and misshaped/mismatched laser beam spots incident on opposite sides of the workpiece.
In a preferred form, the optimal processing is characterized by the application of a first laser beam spot to one side of the workpiece and the application of a second laser beam spot to another side of the workpiece, wherein (i) the energy density of the first laser beam spot is substantially equal to the energy density of the second laser beam spot, (ii) the respective sizes and shapes (i.e., areas) of the first laser beam spot and second laser beam spot are substantially equal to one another, and (iii) the respective impact areas represented by the first laser beam spot and second laser beam spot are disposed substantially opposite one another. The same conditions apply when a pattern of spots is desired. Attaining these conditions results in optimal laser shock processing of the workpiece.
However, in view of the fact that the specific selection of laser peening parameters (e.g., spacing between spots, angle of incidence, distance of spots to certain edges) is made in relation to an ideal workpiece that may vary in its construction (i.e., dimensions and geometrical features) from the actual workpiece (namely at the current target area of interest), the ability of the laser shock processing treatment to maintain substantially constant energy densities at opposing sides of the workpiece is compromised due to the potential dissimilarities between the actual and ideal workpieces. Additionally, any fixturing misalignments and part distortion introduced during laser shock processing will also contribute to the difficulty in maintaining proper energy density levels.
Accordingly, even though the workpiece may be moved during processing to precisely track the sequence of positions defined by the part program, the spatial relationship between the intended target areas and the laser beam path is being adversely modified due to the presence of distortions, surface geometry irregularities, and other dissimilarities and variations between the actual and ideal workpieces. As will be discussed herein, the present invention enables the detection of such dissimilarities and distortions in the actual workpiece and provides a position control mechanism that repositions the workpiece such that the current target area is maneuvered into an adjusted position that substantially matches the ideal target area position, thereby reestablishing the original spatial relationship between the laser beam and target area upon which the original part program was developed.
As used herein, a spatial parameter refers to any characteristic of the workpiece that is suitable for, or capable of, measuring or otherwise determining any variations between the position, geometry, or other spatial feature of any selected area of the workpiece (e.g., intended laser peening target area) and a reference position, geometry, or other spatial feature, such as the relevant characteristics which pertain to an ideal workpiece. The spatial parameter must be such as to afford the possibility of enabling the workpiece to be repositioned such that the target area can be accordingly displaced into an adjusted position substantially matching the ideal position defined by the reference data.
For example, the spatial parameter for the actual workpiece can be the measured distance between a feature of the actual workpiece and expected position of the same feature of the actual workpiece, i.e. where the part would be positioned if it were an ideal workpiece. More directly stated, a measurement is made that represents the spatial orientation of the part and the spatial parameter associated with the measured spatial orientation of the actual workpiece is compared to where the actual workpiece is supposed to be. Where the workpiece is xe2x80x9csupposed to bexe2x80x9d can be determined by empirical measurement of a representative workpiece, but, preferably, is determined through the design of the target area locations on the ideal part. Because the coordinates of the target areas are fixed for a part prior to laser peening and the path of the laser beam is fixed in space, all that is needed is to know where the actual part is positioned with respect to the coordinates of the target areas.
The invention, in one form thereof, is directed to a method of processing a workpiece. According to the method, a workpiece is positioned at a current processing position. Position data is generated that defines at least one spatial parameter that characterizes a positional arrangement of a current target area of the workpiece, wherein the current target area is associated with the current processing position. The position data is processed to evaluate the spatial parameter in relation to predetermined criteria. The position of the workpiece is adjusted in accordance with the evaluation results. Laser shock processing is then performed on the workpiece at the current target area following the position adjustment step. In a preferred form, the steps of the workpiece processing method are repeated for each respective position of a predetermined sequence of positions, such as those of a fixed part program.
The spatial parameter, according to one form thereof, defines a target distance measurement representing the distance between the current target area of the workpiece and a reference point. This target distance measurement is then compared to a predetermined distance value, with the comparison result being used to adjust the position of the workpiece.
Similarly, one or more spatial parameters could define the measured position and orientation of a feature of the workpiece, representing the changes in the position and orientation (distances and angles) between the current location and orientation of the feature and the reference values for the feature. The position and orientation measurements are then compared to predetermined position and orientation values, with the comparison result being used to adjust the position of the workpiece.
The workpiece preferably corresponds to an integrally bladed rotor or other gas turbine engine component. More specifically, the current target area of the workpiece preferably includes at least one of a leading edge section and a trailing edge section of an airfoil in the integrally bladed rotor.
The data processing step, according to one form thereof, further includes evaluating the spatial parameter to calculate a possible shift in the position of the workpiece from the current processing position which would be sufficient to enable optimal laser shock processing of the workpiece at the current target area. This optimal laser shock processing, according to a preferred form thereof for two-sided processing, involves applying a first energy signal having an energy density to a first impact area of the workpiece and applying a second energy signal having an energy density substantially equal to the energy density of the first energy signal to a second impact area of the workpiece, wherein the first impact area and the second impact area are substantially equal and are disposed substantially opposite to one another.
The invention, in another form thereof, is directed to a method of processing a workpiece. According to the method, a part program is provided which defines a plurality of sequential processing positions. The workpiece is positioned at a current one of the sequential processing positions. Position data is provided that defines at least one spatial parameter which characterizes a positional arrangement of a current target area of the workpiece, wherein the current target area is respectively associated with the current processing position. The position data is processed to evaluate the spatial parameter in relation to predetermined criteria. The position of the workpiece is adjusted in accordance with the evaluation results. The workpiece is laser shock processed at the current target area following the position adjustment step. The workpiece is then positioned at a next current one of the sequential processing positions, and the foregoing steps are repeated until the part program is finished.
The invention, in another form thereof, is directed to a method of processing a workpiece. According to the method, the workpiece is positioned at a current processing position, wherein the current processing position is associated with a current target area of the workpiece, and the current target area of the workpiece has a target position value associated therewith that represents the position thereof. A difference measurement is generated which indicates the variation of the position of the current target area of the workpiece from a reference target position, using the target position value. The position of the workpiece is adjusted in accordance with the difference measurement so as to enable the position of the current target area, following positional adjustment of the workpiece, to substantially match the reference target position. Laser shock processing of the workpiece is performed at the current target area following the position adjustment step.
The invention, in another form thereof, is directed to a method of processing a workpiece. According to the method, a workpiece is positioned at a current processing position. Position data is provided that defines at least one spatial parameter which characterizes a positional arrangement of a current target area of the workpiece, wherein the current target area is associated with the current processing position. The position data is processed to generate position adjustment data based thereon which represents a possible displacement of the workpiece from the current processing position to another position where optimal laser shock processing of the current target area can occur. The position of the workpiece is adjusted in accordance with the generated position adjustment data. The workpiece is then laser shock processed at the current target area following the position adjustment step.
The invention, in yet another form thereof, is directed to a method of processing a workpiece. According to the method, the workpiece is positioned at a current processing position. Position data is generated that defines at least one spatial parameter which characterizes a positional arrangement of a current target area of the workpiece, wherein the current target area is associated with the current processing position. The generated position data is compared to predetermined reference data. The position of the workpiece is then adjusted in accordance with the comparison results. Laser shock processing of the workpiece is performed at the current target area following the position adjustment step.
The invention, in yet another form thereof, is directed to a method of processing a workpiece. The workpiece is positioned at a current processing position, wherein the current processing position is associated with a current target area of the workpiece. A determination is made of a position adjustment for the workpiece from the current processing position to an adjusted processing position which would be effective in arranging the workpiece so as to enable the current target area of the workpiece to undergo laser shock processing satisfying predetermined criteria. The position of the workpiece is then adjusted in accordance with the position adjustment determination. The workpiece is laser shock processed at the current target area following the position adjustment step.
One advantage of the present invention is that the laser shock processing treatment is not subject to the limitations that attend the hard-coded positioning of conventional part programs since dynamic feedback enables the current target area to be dynamically repositioned for optimal processing.
Another advantage of the present invention is that the workpiece can be continuously evaluated on a shot-to-shot basis to ensure that each laser firing repetition produces optimal processing of the workpiece or otherwise satisfies a selected performance criteria.
A further advantage of the present invention is that the laser shock processing treatment is made more efficient by ensuring that the laser hits the intended target area.
A further advantage of the present invention is that the otherwise deleterious effects of distortion and deformation are ameliorated by dynamically repositioning the workpiece in response to the detection of such distortive features, thereby serving to compensate for the presence of the distortion.